1. Field of the Invention
The present invention relates to a high frequency heating apparatus which uses microwaves, for heating food or a dielectric of, e.g. a catalyst, and more particularly to a high frequency heating apparatus which utilizes an inverter power supply for driving a magnetron which generates microwaves.
2. Description of the Related Art
The construction of a related high frequency heating apparatus is described with reference to the circuit diagram thereof shown in FIG. 8. In the figure, power from a commercial power supply 1 is converted into direct current by a rectifier 2. The DC voltage is applied through a filter circuit 3 to a resonance circuit composed of a capacitor 4 and an inductor 5 and a series circuit composed of a semiconductor switching device 6 and a diode 6A. The semiconductor switching device 6 oscillates at a frequency of several tens kHz or more to generate high frequency alternating current, working together with the resonance circuit. The voltage of the alternating current generated in the inductor 5 is raised by a transformer 7, whose primary winding is the inductor 5. The high voltage provided by the transformer 7 is converted into a DC high voltage by a high-voltage rectifier 8. A control circuit 9 signals to drive the semiconductor switching device 6. These electric component parts thus compose an inverter power supply (a power converter) 10. The DC high voltage provided by the high-voltage rectifier 8 is applied between the anode and cathode of a magnetron 11. The transformer 7 is provided with an extra winding 12 which supplies power to the cathode of the magnetron 11. When the cathode is heated by the power supplied thereto and the high voltage is applied between the cathode and anode, the magnetron 11 oscillates to generate microwaves. The microwaves thus generated are used to irradiate an object, such as food, placed in a heating chamber.
Since the inverter power supply 10 processes high power such as 1 to 2 kW, the electric component parts thereof cause a substantial loss and which is dissipated as heat. Therefore, the electric component parts must be cooled. For example, the inverter power supply 10 is provided with forced-air cooling means composed of a motor 13 and a fan 14, which flows air to cool the electric component parts. The rectifier 2 and the semiconductor switching device 6 are provided with aluminium fins to facilitate heat radiation.
FIG. 9 illustrates a high frequency heating apparatus body 15 to which an inverter power supply 10, a magnetron 11, a motor 13 and a fan 14 are separately mounted. As understood from the illustration, the air stream must cover a substantially large area in order to sufficiently air-cool the inverter power supply 10 and the magnetron 11. Therefore, a propeller fan is employed as the cooling fan 14, which can generate a large air flow. An AC motor is employed as the motor 13 to drive the cooling fan 14. Thus, the forced air cooling is performed by a combination of an AC motor and a propeller fan. Such an air cooling system becomes inevitably large.
Such a conventional high frequency heating apparatus has problems as described below.
First, since the inverter power supply 10, the motor 13, etc., are separately mounted to the high frequency heating apparatus body 15, many assembly processes are required. Lead wires must be used to connect components such as the inverter power supply 10, the motor 13 and the like to the power source in order to supply required powers to the components respectively. During assembly, after the inverter power supply 10 and the motor 13 are mounted to the high frequency heating apparatus body 15, they are connected to the power source by the lead wires. Since there is only a small space for the lead wires to be wired, the wiring process is not easy and normally requires manual labor. Also, since the inverter power supply 10, the motor 13, etc., vary in shape and each of them must be wired with lead wires, the assembly processes are hard to automate and simplify.
Second, the lead wires supplying power to the inverter power supply 10, the motor 13, the magnetron 11, etc., radiate undesirable electromagnetic waves which affect electrical appliances, such as a TV or a radio, placed nearby.
Third, since the semiconductor switching device 6 produces a lot of heat and requires substantially large fins for efficient heat radiation, the large fins take up a large amount of space on the printed board, and thus hinder employing a small and compact printed board.
In addition, U.S. Pat. No. 4,956,531 discloses a power module in which an inverter power supply is placed in a metallic envelope and a magnetron and a fan are compactly combined. In the above power module, the three components are separately placed in different casings. Therefore, the three casings must be connected to one another during the assembly of the high frequency heating apparatus. Further, the casing of the magnetron and the casing of the inverter power supply must be connected to the power source by means of lead wires. This wiring process is troublesome Also, the lead wires used for the connection are likely to radiate undesirable electromagnetic waves (noises).